A duck-billed platypus. If you were mystified at first, you are in good company: the scientists were mystified, too. However, this is how they solved the puzzle: the animal has fur and feeds its young on milk. This makes it a mammal, but a rather unusual one. It forced the creation of a subset of mammals called monotremes.

It could not be a fish, amphibian, reptile, or bird, because it has fur. It has a backbone, so it could only be a mammal. Other characteristics of anatomy, such as arms and legs, pelvis, kidney structure, heart chambers, and other features are also shared only with mammals.